"that far outweighs any potential benefit" I disagree. This question has a vote score of +56/-0, while the original (posted half a year earlier) has only +28/-0. Why should the duplicate poster gain twice as much rep as the original asker? Doesn't this encourage a lack of research?
–
WCharginJun 6 '13 at 18:46

Closing is more transient than votes are. As you mentioned, mistakes do happen. And on the flip side we can't ever guarantee that the dupe author will return or will confirm it even if it really is a duplicate (be it stubbornness, spite, or ignorance). What about if the author does confirm, but people vote and reopen it anyway? And how would it handle when a question is closed as a duplicate of multiple questions? Or when people who voted on the duplicate already voted on the target(s)?

It's going to be a lot of effort and calculation for a system that I don't see having a large valuable usage.